The present invention relates to signal pickup devices for reproducing recorded signals from rotary recording mediums. More particularly, the invention relates to a video signal pickup device capable of tracking control. The pickup is used primarily in an apparatus for reproducing video signals from a rotary disc on which the signal has been recorded on either a spiral track or concentric circular tracks. The tracking control causes the reproducing stylus to accurately trace over the recorded track. Further, there is a jitter compensation in a manner such that a reproducing stylus moves along the longitudinal direction of the track.
Heretofore, there have been apparatus of the designed type. For example, a rotary disc (referred to as "disc" hereinafter) has a video signal recorded on a spiral track as variations in the geometrical shapes corresponding to an information content. A reproducing transducer, that is, a signal pickup device is caused to trace over the spiral track and reproduce the recorded video signal. In a pickup device of this character, it is necessary for the signal pickup device to trace the track accurately. For this reason, it is necessary to provide means for detecting any tracking deviation of the signal pickup device relative to the track. In response to this error, the position of the signal pickup device is controlled so that it will trace accurately over the track thereby accomplishing a tracking control.
The present applicant has previously described in a commonly assigned United States Patent Application Ser. No. 841,531, filed Oct. 11, 1977, by Kazuo Tatsuguchi and Hisao Kinjo, and entitled "Reproducing transducer capable of tracking control", a signal pickup device in which a cantilever or a conductor fixed thereto is disposed within a magnetic field created by a permanent magnet. The pickup is supplied with the current of a tracking control signal. Responsive thereto, the cantilever undergoes a displacement in response to the direction and magnitude of the control current to cause a reproducing stylus to trace at a correct position along the recording track on the recording medium.
This signal pickup device, however, is accompanied by various problems due to its construction, the most serious of which are recited below.
1. The length of the cantilever arm or the conductor is short in the magnetic field due to the permanent magnet. Therefore, a sufficient displacing force cannot be produced for accomplishing a tracking control.
2. The cantilever position easily varies within the magnetic field because of disc surface oscillation and the like. The magnetic flux density (or magnetic induction) varies at the position of the cantilever thereby giving rise to a change in the tracking gain characteristic. As a result, an optimum control cannot be achieved, and accurate and positive tracking control can no longer be accomplished.
3. Since the lead wires are signal detection and the conductor for tracking control are in close proximity to each other, a mutual induction takes place, whereby the signal reproducing characteristic and tracking accuracy deteriorate.